With the proliferation of mobile electronic devices in recent years, development for a lithium ion secondary battery with high energy density and high output has been proceeding. However, since a lithium ion secondary battery contains an electrolyte comprising a non-aqueous solvent and a lithium salt dissolved therein, when a large current flows across the battery due to a short circuit or rupture, or the battery is overcharged, the battery temperature may rapidly increase, leading to thermal runaway.
In order to obviate this, a protective circuit (hereinafter referred to as “SU (safety unit) circuit”) is installed, or a current interrupting device that operates in the case of the breakdown of the SU circuit, such as a PTC (Positive Temperature Coefficient) device or a thermal fuse, is used as backup in a battery pack that houses the lithium ion secondary battery. However, since the SU circuit is expensive and its structure is complicated, demand is increasing for an inexpensive circuit with a simple structure.
At the same time, there is proposed a simple-structured current interrupting device comprising a recoverable device and a non-recoverable device connected in series with each other, and its weight and size reduction is also proceeding (see Japanese Laid-Open Patent No. 2002-150918).
The recoverable devices include a PTC device, a shape memory alloy device (hereinafter referred to as “SMA device”) (see Japanese Laid-Open Patent Publication No. 2001-283828), etc, all of which have functions to first break a current and then restore the current.
The non-recoverable devices include a thermal fuse and a pattern fuse, both of which completely break a current in the case of the breakdown of the recoverable device and function as the backup for the recoverable device.
When the battery is suddenly exposed to harsh temperature conditions or the like, however, the non-recoverable device may operate prior to the recoverable device, depending on the battery atmosphere to be used. In such cases, a defect that the battery can no longer be used arises although it does not negatively affect the battery performance. In order to prevent this, the operating temperature of the conventional non-recoverable device is usually set to not less than 150° C. This can prevent the problem that the non-recoverable device operates prior to the recoverable device, but thermal runaway may not be prevented by the non-recoverable device when the operating temperature of the non-recoverable device is set to such a high temperature.
Accordingly, a method for enhancing the thermal resistance of an overcharged battery by optimizing a positive electrode material and a non-aqueous electrolyte is conceived. However, once the battery temperature is increased to about 150° C., the optimization effect is diminished and thermal runaway may not be prevented.
In the case of a lithium ion secondary battery, once the battery temperature is increased to not less than 90° C., the battery will not exhibit its full performance even if the battery is used again. Therefore, even if the non-recoverable device operates prior to the recoverable device, it is presumed that there is no actual damage caused.